This invention relates to nondestructive testing of conductors for constrictions and discontinuations, and particularly for testing of thin conductors located on highly thermal conducting substrates.
A significant and increasingly difficult problem with packaging and interconnection systems in complex circuits, such as circuits in computers with high levels of integration, is the detection of incipient opens that escape standard conductivity testing of conductors.
In order to test thin conductors located on highly thermal conducting substrates by means of a non-linear conduction method, a higher operating frequency is required than has been the case heretofore. The higher operating frequency is required because of the much shorter thermal time constants encountered.
As used herein the term thin conductors generally refers to a conductor having physical dimensions less than 1 mil by 1 mil in cross-section disposed on a substrate. The term highly thermal conductive substrate as used herein refers to substrate materials such as ceramics and quartz and other substrates having a thermal conductivity in the range between approximately 0.003 to 0.03 watt/cm .degree.C.
A prior art non-linear detection technique is described in U.S. Pat. No. 4,496,900, assigned to the assignee of the present invention, and incorporated herein by reference. The patent concerns defect detection on glass-epoxy printed circuit boards where the optimum detector operating frequency is one kilohertz. The prior art technique is limited to testing conductors located on substrates which have longer thermal time constants than are encountered with many currently available printed circuit boards or thin film substrates.
In U.S. Pat. No. 4,404,514 there is shown a system for detecting and locating faulty connections in a communication system. A test signal containing the sum of two respective output signals from a pair of oscillators operating at different frequencies in the range between 50 and 400 MHz is transmitted into a cable. A defect in the cable causes a beat frequency signal to be generated along the cable. The time between the transmission of the test signals and receipt of the defect induced signal is indicative of the defect location along the cable.
In the present invention, a test signal containing a direct current offset signal combined with two alternating current signals at different operating frequencies in the range between 10 kilohertz and 20 megahertz is applied to a conductor under test. Upon encountering a defect, intermodulation signals are generated. The phase of the difference frequency intermodulation signal is compared with the phase of a reference signal to indicate the presence of a defect. The mere existence of a signal is not sufficient, rather the phase shift of the signal is indicative of the presence of a defect. Moreover, the frequencies of the two alternating current signal components is important to assure the sensing of defects of the type being sought in the thermal conductivity environment in which the conductors are embedded.